This invention relates to balloon catheters which are especially useful in medical dilatation procedures.
Balloon catheters have been found to be useful for the relief of arterial stenosis as in percutaneous transluminal angioplasty (PTA) and percutaneous transluminal coronary angioplasty (PTCA) procedures and in many other medical applications involving not only insertion into blood vessels but also involving insertion into a variety of body cavities. Balloons can be made from a variety of commercially available materials which are generally of the thermoplastic polymeric type. Included among the known materials are polyolefins, poly(vinyl chloride), polyethylene terephthalate, polyamides, polyetheramides and the like. Balloons made from such materials exhibit the properties of the materials from which they are made. For example, toughness, flexibility, tensile strength, and elasticity are properties which can be produced in balloons from off-the-shelf polymeric materials. For example, polyamides were thoroughly characterized in their structure and properties and disclosed in various patents of the DuPont company in the late 1930's and early 1940's. One such property is their ability to make high strength, thin-walled balloon structures by blowing and stretching a tube in a mold to produce biaxial orientation of the material as disclosed in U.S. Pat. No. 2,307,817 issued to Austin. The same balloon materials have been used to make balloons for medical catheters as disclosed in U.S. Pat. No. Re. 32,983 issued to Levy or U.S. Pat. No. 5,055,024 issued to Jackowski et al.
The specific processing steps required to make such biaxially oriented products is well established in the art of plastic bottle making. For example, in U.S. Pat. No. 4,721,654 issued to Richardson et al, a parison of a copolyamide material is subjected to stretching and internal pressure in a mold by which the material is radially and longitudinally stretched to produce a biaxial orientation. The molded product is then annealed in the mold at its final shape to increase the crystallinity of the material. This is accomplished by maintaining pressure inside the molded article and contacting the article against the heated mold for the time required to heat set the material. In the making of a medical balloon, this process may be adopted with various modifications and additional steps required to produce a suitable balloon shape. For example, in U.S. Pat. No. 5,334,146 issued to Ozasa, a catheter balloon is made by forming a tubular parison made of a drawable polymer, heating the parison at a temperature in the range from the second-order transition temperature to the first-order transition temperature of the polymer used, and then stretching it in the direction of its axis and then inflating it radially while heated. The stretched and inflated parison is then cooled below the second-order transition temperature of the polymer, and then deflated. The resulting balloon has a cylindrical portion of a substantially uniform desired diameter and desired wall thickness but tapered portions of a wall thicknesses which are thicker than desired thicknesses at the front and rear of the cylindrical portion and at the connecting portions at the front and the rear of the tapered portions. Therefore, the balloon is subjected to a redrawing operation in the tapered portions of the balloon to reduce their wall thicknesses to desired thicknesses by stretching the tapered portions in the direction of the axis of the balloon. However, the disclosed method of redrawing requires additional processing steps which are undesirable in a mass-production balloon. It is therefore an object of the invention to form the balloon in a single molding process in which the cylindrical portion of the balloon, the tapered portions of the balloon and the connecting portions of the balloon are all formed in a single molding operation which makes the balloon ready for attachment to the catheter.